Why can I not import the name ModbusResponse from pymodbus.pdu?

The reason you cannot directly import ModbusResponse from pymodbus.pdu is because it's not directly exposed as a name available for import in that specific module. The pymodbus.pdu module primarily defines the Protocol Data Unit (PDU) structure, which is the core data structure for Modbus communication. Instead of directly importing ModbusResponse, you typically work with specific response classes that inherit from a base response class that might be defined within the library's internal structure.

Here's a more detailed explanation:

1. Module Structure: pymodbus is organized into submodules. pymodbus.pdu deals with the low-level structure of Modbus messages. The actual response handling is typically done in other parts of the library like the client or server modules.

2. Specific Response Classes: Instead of a generic ModbusResponse, pymodbus uses specific response classes for each function code (e.g., ReadHoldingRegistersResponse, WriteSingleCoilResponse). These are created when a Modbus request is processed and contain the data associated with that response.

3. How to use them: When you make a Modbus request through a pymodbus client, the response you receive is an instance of one of these specific response classes. You then interact with the response objects through their specific attributes and methods.

4. Example Clarification If you look at the pymodbus library, you will not find a class called exactly ModbusResponse in the module pymodbus.pdu, Instead, you would see classes like ReadHoldingRegistersResponse, WriteSingleCoilResponse, etc. These are the specific classes that are created when Modbus data is received and parsed. It's these classes which are returned when calling, for example, the read_holding_registers client method.

5. Correct Usage Pattern: You typically don't import response classes directly from pymodbus.pdu. Instead, you use the client functions (e.g., read_holding_registers) and handle the response objects these functions return. This might look something like this:

from pymodbus.client import ModbusTcpClient
client = ModbusTcpClient('127.0.0.1')
client.connect()
response = client.read_holding_registers(address=0, count=10, unit=1)
if response.isError():
  print("Error in response:", response)
else:
  print("Registers:", response.registers)
client.close()

In short, the design of pymodbus means you don't import a generic ModbusResponse from pymodbus.pdu. Instead, the response to Modbus requests is encapsulated by specific classes (like ReadHoldingRegistersResponse) that the library's client methods return. This structured approach is more type-safe and makes it easier to work with different types of Modbus messages.

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